JK1991/iStock/Thinkstock(NEW YORK) — The U.S. Food and Drug Administration issued warning letters to four companies on Wednesday that sell products online that claim to “prevent, diagnose, treat, or cure cancer without evidence to support these outcomes.”

According to the FDA, the sale of unapproved products can put patients at risk, and “the deceptive marketing of unproven treatments may keep some patients from accessing appropriate, recognized therapies to treat serious and even fatal diseases.”

The agency says it is concerned by the sale of such products, which allegedly contain cannabidiol, a component of the marijuana plant. Cannabidiol is not approved for any indication, the FDA says, but is marketed in a variety of products that claim to reverse or cure cancer, kill or inhibit cancer cells, and other anti-cancer claims.

“Substances that contain components of marijuana will be treated like any other products that make unproven claims to shrink cancer tumors,” said FDA Commissioner Scott Gottlieb. “We don’t let companies market products that deliberately prey on sick people with baseless claims that their substance can shrink or cure cancer and we’re not going to look the other way on enforcing these principles when it comes to marijuana-containing products.”

The letter was sent to Greenroads Health, Natural Alchemist, That’s Natural! Marketing and Consulting, and Stanley Brothers Social Enterprises LLC.

“We have an obligation to provide caregivers and patients with the confidence that drugs making cancer treatment claims have been carefully evaluated for safety, efficacy, and quality, and are monitored by the FDA once they’re on the market,” Gottlieb said.